Gallium nitride (GaN) based solid-state lighting technologies have made advances in the past three decades and promise to replace traditional fluorescent and incandescent bulbs for general illumination at tremendous energy and cost savings. GaN-based high-brightness light-emitting diodes (LEDs) have already penetrated various markets such as displays, traffic signals, cell phone backlights, and automotive lighting, and white LEDs have presence in various niche lighting markets. The efficiencies of such commercially available white LEDs have surpassed that of incandescent lighting and are now competitive with that of fluorescent lamps (˜70-100 lumens per Watt (lm/W)). However, the performance of individual LED components still falls short of what is required to enable a ubiquitous general illumination solution. A source efficiency of 150 lm/W surpasses the majority of traditional lighting technologies, and a white light source with this efficiency is widely considered to be a viable replacement for conventional light sources. Thus, significant improvements are still required, and improving the efficiency of GaN-based LEDs for such applications remains the subject of vigorous research efforts.
There are two main components to the overall efficiency of an LED: the internal quantum efficiency (IQE) and the external quantum efficiency (EQE). IQE is related to the efficiency of the epitaxial device structure itself. EQE is the product of IQE and what is known as the extraction efficiency, and represents the total light out of a packaged device. Improved extraction efficiency (and, consequently, improved EQE) has become an important aspect of conventional LED manufacturing, in particular because there are limitations to the IQE which can be achieved in conventional devices.
One limiting factor is a result of the polar c-plane crystallographic orientation of conventional GaN-based devices. Strong internal electric fields are present in this direction which are well-known to reduce radiative recombination efficiency in optical devices due to spatial separation of electron and hole wave functions. One method to eliminate such effects is to grow on alternate orientations of GaN (for example orientations commonly known as “nonpolar” or “semipolar”) in which such internal electric fields are either absent or greatly reduced. Several recent device demonstrations in various nonpolar and semipolar orientationsi,ii have validated this approach for reducing internal fields and increasing IQE in LEDs.
Additionally limiting the IQE of conventional devices is the extremely high dislocation densities which arise from the use of foreign substrates such as (0001) sapphire or (0001) silicon carbide to grow GaN-based materials, a process commonly known as “heteroepitaxy”. Heteroepitaxy results in GaN films with extremely high extended defect densities (e.g. threading dislocation (TD) densities of ˜109-1010 cm−2). TDs in GaN are widely recognized to act as nonradiative recombination centers, thus reducing the internal quantum efficiency of heteroepitaxial devices. TDs can also provide significant leakage paths in diodes. The presence of basal plane stacking faults (which are much more prevalent in heteroepitaxy of nonpolar orientations of GaN) can also promote faceting, particularly during the growth of the active region of LEDs.
While improved growth techniques such as lateral epitaxial overgrowth (LEO) have been successful at reducing overall TD densities in polar and nonpolar GaN films, and have resulted in modest improvement in device performance in recent years, these approaches are processing-intensive and cost-prohibitive for LEDs. The most promising approach is to grow on high-quality native bulk-GaN substrates, thus mitigating dislocation generation due to heteroepitaxy. While there remains an extremely limited supply of bulk-GaN material and in particular cost-effective material suitable for use as a substrate, bulk-GaN material can be obtained. For example, pseudo-bulk-GaN material can be obtained by slicing the desired material orientation from a thick GaN sample grown heteroepitaxially (for example by hydride vapor phase epitaxy) and promising device demonstrations on such material have been shown in the literature.iii,iv,v,vi,vii Certain progress has also been made in the area of bulk-GaN synthesis by ammonothermal techniques as described in, for exampleviii M. P. D'Evelyn, H. C. Hong, D.-S. Park, H. Lu, E. Kaminsky, R. R. Melkote, P. Perlin, M. Lesczynski, S. Porowski, R. J. Molnar, “Bulk GaN crystal growth by the high-pressure ammonothermal method”, Journal of Crystal Growth 300, 11-16 (2007).
It can be recognized from the above discussion that there are limitations to the IQE achievable in conventional GaN-based devices, and in particular LEDs, which can be significantly improved or overcome with the use of bulk-GaN substrates. However, as bulk-GaN technology is in its infancy, conventional LED manufacturing continues to employ heteroepitaxially grown structures. Therefore improvements in extraction efficiency have been made which can mitigate loss due to absorption in the substrate or contacts, bond pads, wire bonds, among other sources of loss. Dramatic improvements in conventional heteroepitaxial devices have been realized in recent years from several industry leaders by implementing LED devices and geometries known as “high-extraction” structures. An example of such a structure aimed at high extraction efficiency was shown in the literature by Fuji et al., and the representative schematic is shown in FIG. 1.ix 
A main component of such high-extraction structures is the inverted orientation. The device is flipped over and mounted to a submount such that light extraction occurs through the backside, or n-doped side, of the LED. Also of significance is the short optical cavity length. For conventional heteroepitaxial LEDs in this type of configuration, a short optical cavity length can be achieved by separating the thin GaN epilayer from the non-native substrate using a method such as laser lift-off (in the case of sapphire), or dry- or wet-etching (in the case of SiC). These methods rely on a difference in either a physical property (band-gap) or chemical property (reactivity) to effect a separation at the substrate/epilayer heterointerface. During a sapphire laser lift-off process, a high-power laser beam (with a characteristic energy lower than the bandgap energy of the sapphire and higher than the bandgap energy of GaN) is directed at the GaN/sapphire interface through the optically transparent sapphire substrate. The laser energy induces the decomposition of the highly defective GaN at the GaN/sapphire interface into elemental nitrogen and gallium, resulting in spontaneous separation of the substrate and the epilayer. In the case of LEDs on SiC, there exist etch chemistries with sufficiently high etch rates as well as good etch selectivity (between SiC and GaN) to enable effective removal of the SiC substrate. For LEDs on bulk-GaN, however, the substrate and epilayer have substantially the same physical and chemical properties. Thus, effecting a separation based on the conventional methodologies just described is not possible.
As for the background, the benefits of such a high-extraction LED device structure and geometry for conventional heteroepitaxial LED devices can be known from the literature. For example, Philips Lumileds Lighting has reported that heteroepitaxial devices which include a short optical cavity length LED device and a roughened extraction surface exhibit superior performance relative to heteroepitaxial devices which emit light through a sapphire substrate.x Such results are shown in FIG. 2 (reproduced from Ref 11) in which the LEDs being compared were implemented into YAG:Ce phosphor-coated white LED lamps. The lamp performance is plotted in terms of luminous efficiency (lumens per Watt, lm/W) and flux (lumens). It can be seen that the light output of the thin-film flip-chip LED-based lamp is ˜45% higher than that of the conventional flip-chip LED-based lamp over the entire range of current shown.
From the above, it can be seen that the use of bulk-GaN substrates is highly desired in order to improve upon the efficiency, in particular the IQE, of GaN-based devices. Additionally, device structures and geometries which are aimed at high-extraction are necessary and currently utilized in the industry for improved EQE in GaN-based devices.